There has been proposed to execute power transmission to a cordless equipment by making use of mutual induction (electromagnetic induction) by an induction coil for the purpose of charging a secondary battery such as a nickel-cadmium battery or a lithium ion battery incorporated in the cordless equipment such as a cordless handset or a portable telephone set, and the scheme as described above is disclosed, for instance, in Japanese Patent Laid-Open Publication No. SHO 54-12422, Japanese Patent Laid-Open Publication No. HEI 5-64375, Japanese Patent Laid-Open Publication No. HEI 5-300662, and Japanese Patent Laid-Open Publication No. HEI 7-170312.
In a noncontact charging device for executing power transmission by making use of mutual induction, a primary side induction coil is provided in a charger, a secondary side induction coil is provided in a cordless equipment, an LC resonance circuit is formed in the charger as well as in the cordless equipment with a combination with a capacitor, and power is transmitted to the cordless equipment from the charger by making use of electromagnetic induction under high frequency oscillation.
In order to efficiently transmit power by making use of mutual induction, it is necessary to set a degree of magnetic connection between a primary side induction coil and a secondary side induction to an appropriate value under high frequency resonance, and in order to set the degree of electromagnetic connection to an appropriate degree, it is required to satisfy the prerequisites not only that the cordless equipment must be positioned against the charger when charged, but also that a mounting position for the coil in each of the charger and cordless equipment has been set with high precision.
In other words, when charging, the primary side induction coil and the secondary side induction coil must be magnetically connected at a prespecified gap to each other so that they are connected to each other with a high magnetic flux density atmosphere, and also it is desired that the magnetic connection area is large, and otherwise waste of electric power disadvantageously increases.
However, a primary side induction coil and a secondary side induction coil in a conventional type of noncontact charging device are wound coils, and for this reason it is difficult to set mounting positions for the induction coils in a charger as well as in a cordless equipment with high precision, and also the construction for mounting is complicated. In a case of a wound coil, if the wound coil is one having a specific form with a core, precision in the mounting position for an induction coil is high, but in this case construction of the induction coil is complicated and the cost is high.
Also the magnetic connection area is enlarged when a winding diameter or the number of turns of winding increases, but this makes size and weight of the induction coil larger, and it is hard to use this type of induction coil in a cordless equipment with portability such as a portable telephone set which is desired to be less in size and weight.
Also in a noncontact charging device based on the mutual induction system as described above, even in a state where a cordless equipment has not been set in a charger (in a stand-by state), if a metallic conductor such as coin is placed on a charger while the primary side induction coil in the charger is oscillating at the same output level as that in charging, an eddy current flows in the metallic conductor due to a magnetic field caused by the primary side induction coil, which generates induction heating and increase in wasteful power consumption.